What is renewable energy? Solar, wind, water and more

Renewable energy is energy from sources that nature refills faster than people use them up: sunlight, wind, flowing water, the heat inside Earth and plants that regrow. Coal, oil and gas are the opposite. They took millions of years to form and are burned in moments. In 2025, renewables made about a third of the world's electricity, and they are the fastest-growing source of new power.

In short

• "Renewable" is about the source being refilled, not about being clean: most renewables emit little carbon, but the two ideas are different.
• Solar and wind now make up almost all new power capacity, but they only produce when the Sun shines or the wind blows, which is why storage and grids matter.
• A power plant's size on paper is not what it produces: a solar farm makes far less in a year than its rating suggests, and that gap shapes every comparison.

Which sources count as renewable, and how does each make electricity?

Five sources make nearly all renewable power. Most of them, in the end, are sunlight in another form: the Sun heats air unevenly, which makes wind, and lifts water as vapor that falls as rain on high ground, which fills rivers.

Source	How it makes electricity	What it depends on
Solar photovoltaic	Light knocks electrons loose in silicon cells, making a direct current	Daylight, season, cloud
Wind	Moving air turns blades, which spin a generator	Wind speed, which varies by hour and season
Hydropower	Water falling through a dam spins a turbine	Rainfall and snowmelt; droughts cut it
Geothermal	Hot water or steam from underground drives a turbine	Hot rock near the surface, in a few regions
Biomass	Wood, crop waste or gas from rotting waste is burned to raise steam	Plants being regrown at the rate they are burned

Ocean energy, from waves and tides, works on the same turbine principle but is still at an early stage, according to the United Nations' overview of renewable energy. Renewables also supply heat and fuel, such as solar water heaters and biofuels in cars, not just electricity.

Is nuclear power renewable?

No. Nuclear power splits uranium, a metal mined from the ground in finite amounts, so it isn't renewable. It is low-carbon, which is why energy analysts often group it with renewables as "clean" or "low-carbon" power, and why the two labels shouldn't be swapped. Nuclear fusion, which would fuse hydrogen as the Sun does, would not be renewable in the strict sense either, but its fuel would last so long that the difference hardly matters. It doesn't yet produce power for any grid.

How much of the world's power is renewable?

Renewables made 33.8% of the world's electricity in 2025, slightly more than coal's 33.0%, the first time in about a century that they passed coal, according to Ember's Global Electricity Review 2026; Ember is an energy think tank. Hydropower is still the largest single renewable source, but solar is growing fastest: it made 2,778 terawatt-hours (TWh) in 2025, more than wind for the first time.

The growth is in new building. In 2024 the world added a record 585 gigawatts (GW) of renewable capacity, and renewables were 92.5% of all new power capacity added that year, according to the International Renewable Energy Agency (IRENA). Electricity is only part of the picture, though: heating, transport and industry still run mostly on fossil fuels, so the renewable share of all energy used is well below a third.

Why does a solar farm produce less than its size suggests?

A plant's capacity is the most it can produce at once. Its capacity factor is what it actually produces over a year, as a share of that maximum. The difference is the single most misunderstood thing about renewable power. Here is the arithmetic, using 2025 figures for US plants from the Energy Information Administration (EIA).

A year has 8,760 hours. A 1 GW plant running flat out all year would make 1 GW × 8,760 hours = 8,760 gigawatt-hours (GWh).

1 GW of…	US capacity factor, 2025	Output in a year
Solar photovoltaic	24.0%	about 2,100 GWh
Wind	33.8%	about 2,960 GWh
Hydropower	34.4%	about 3,010 GWh
Geothermal	65.9%	about 5,770 GWh
Nuclear	91.1%	about 7,980 GWh

So to make as much electricity in a year as 1 GW of nuclear, you would need about 7,980 ÷ 2,100 ≈ 3.8 GW of solar, or about 2.7 GW of wind. That is why renewables made 46% of the world's installed capacity at the end of 2024, by IRENA's count, but about a third of its electricity. Neither number is wrong. They measure different things, and comparisons that mix them up mislead.

What happens when the Sun sets or the wind drops?

Solar and wind are variable: they follow the weather, not demand. Demand in many places peaks in the early evening, just as solar output falls away. Grids that run on a lot of variable power use four main tools to cope.

• Storage. Batteries, mostly lithium-ion, charge when power is cheap and plentiful and release it hours later. Pumped hydro does the same at larger scale by pumping water uphill and letting it flow back through turbines.
• Wider grids. The wind is usually blowing somewhere. Long transmission lines let a calm region import from a windy one.
• Flexible plants. Hydropower and gas plants can ramp up quickly to fill gaps.
• Shifting demand. Charging cars, heating water or running factories when power is abundant moves use to match supply.

Batteries cover hours well. The hard case is a week or more of cold, still, cloudy winter weather, sometimes called a "dark doldrum". How best to cover it, with long-duration storage, nuclear, hydrogen or some gas kept in reserve, is one of the main open questions in planning a low-carbon grid.

Why does renewable energy matter?

Burning coal, oil and gas releases carbon dioxide, which strengthens the greenhouse effect and drives climate change. Fossil fuels formed from ancient plants and plankton over millions of years, as the note on how oil is formed explains, so their supply is finite and their price swings with geopolitics. Solar, wind and hydro emit almost nothing while running; their emissions come from making and building the equipment.

Still argued

Not every renewable is equally clean. Burning wood counts as renewable because trees regrow, but the carbon is released at once and taken back up over decades, and cutting forests for fuel can do more harm than good. The UN says bioenergy should be used only in limited cases. Large dams can flood valleys and disrupt rivers. And how much it costs to run a grid on mostly variable power, counting storage and transmission, is argued among engineers and economists, with different studies reaching different answers.

Check yourself

• Why does 1 GW of solar make less electricity in a year than 1 GW of nuclear?
• What makes a source renewable, and why isn't nuclear power one?
• Name two ways a grid can keep the lights on after sunset without fossil fuels.

Sources

• What is renewable energy? — United Nations
• Global Electricity Review 2026 — Ember, 2026
• Renewable Capacity Statistics 2025 — International Renewable Energy Agency, 2025
• Electric Power Monthly, Table 6.07.B: Capacity factors for utility-scale generators not primarily using fossil fuels — U.S. Energy Information Administration
• Renewable energy explained — U.S. Energy Information Administration